Microscopic structure and elasticity of particle gels

Introduction

Many materials consist of random aggregates of microscopic particles, known as particle gels. How does the microscopic arrangement of particles dictate the elastic properties of the gel as a whole? With the confocal microscope, we can completely characterize the architecture of the disordered gel and, in the same experiment, measure its local elastic properties. We want to understand in detail how these are related. The microscope is ideally suited to these experiments because it reveals heterogeneities and topological information.

Gels that differ microscopically share many macroscopic properties, such as fractal dimension, fractal correlation length and a remarkable scaling of the frequency-dependent shear modulus. These experiments, however, show that there are some intriguing differences at the microscopic scale. In gels with a stronger interaction potential, the 'chains' that make up the gel become thinner and more tenuous.

Gel Structure in 3D

A 3D view of gels reveals that the gel is made of chains, not clumps. We measure the topology, shape, and elasticity of these chains using confocal microscopy. The figure at the top of this page is a false-color image of a 20x18x10-micron section of the gel, reconstructed using the measured particle positions. The color code indicates the number of bonds per particle: red particles have 0 or 1 bond, green have 2, blue have 3, yellow have 4 and purple have 5 or more bonds. All of the particles in this image are connected in a single large cluster.

Dynamics of the Gel

The particles in the gel are not stationary; they move due to thermal fluctuations (small kicks from the solvent molecules). By watching these small fluctuations, we can measure the mechanical properties (viscosity, elasticity) of the gel. We are especially interested in seeing how the elasticity scales with length, which will allow us to predict the bulk rheology from microscopic information.

I thank Eric Weeks for teaching me how to make these movies using POV-Ray.

Return